Per previous post.
[snip]
"Parasitics do not cause filaments to break in a tube. Those claims
are silly."
I don't agree.
While trying to source a problem in a single 3-500z amplifier, I observed
a flash and failure of the tube fillament structure. The amplifier was a
homebrew based on the Henry amplifier line and the many Handbook designs.
This amp was plagued by "diode hash" in the standby mode which turned out
to be a parasitic just about 50MHz. This was the second tube to "die" in
the same amplifier within two months.
With an HP 8920 spectrum display and storage counters I was able to
clearly see a strong parasitic oscillation which would actually move back
and forth a few MHz when the front pannel tune/load controls were moved.
The owners reactive antenna load only made the problem much worse
including a flashover which damaged the tube structure. The second tube
still resides on my shelf as a display. I believe it's an Eimac tube.
Many portions of the fillament broke off the helix structure to lay in the
base of the tube as short wires with melted ends.
The big fix was to replace the garbage grid current metering circuit with
a "cathode" return lead type. The grid pins of the 3-500z socket were then
tied directly to the chassis ground through large low XL straps.
A "poor mans" network anal showed bypass caps in the grid pin type
metering circuit had also failed.
The builder of this amp had copied the Henry HF amplifier grid metering
circuit almost "note for note". "This method of grid current metering is
trash" and leads to trouble.
Other additions to the homebrew amplifier were suppressors and some
gain reduction/negative feedback. No stability problems into a reactive or
non-inductive loads were found after the proper changes were made.
A last mention that the HV spacing of the tune and loading caps were to
small in my opinion. This spacing problem might have contributed to
the amount of damage once the "flashover" started.
Although I recomended the owner change these caps out, he still chose to
leave them installed. The amplifier has not experienced any problems of
this type since the modifications were done.
I guess that's living on the margin...
Cheers
Skip May wv6f
mays@indigo.ucdavis.edu
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